Multiple charged-particle beam apparatus with low crosstalk

ABSTRACT

Systems and methods of enhancing imaging resolution by reducing crosstalk between detection elements of a secondary charged-particle detector in a multi-beam apparatus are disclosed. The multi-beam apparatus may comprise an electro-optical system for projecting a plurality of secondary charged-particle beams from a sample onto a charged-particle detector. The electro-optical system may include a first pre-limit aperture plate comprising a first aperture configured to block peripheral charged-particles of the plurality of secondary charged-particle beams, and a beam-limit aperture array comprising a second aperture configured to trim the plurality of secondary charged-particle beams. The charged-particle detector may include a plurality of detection elements, wherein a detection element of the plurality of detection elements is associated with a corresponding trimmed beam of the plurality of secondary charged-particle beams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application 62/853,677 whichwas filed on May 28, 2019, and which is incorporated by reference in itsentirety.

TECHNICAL FIELD

The embodiments provided herein disclose a multi-beam apparatus, andmore particularly a multi-beam charged particle microscope with enhancedimaging signal fidelity using a combination of aperture arraysconfigured to reduce crosstalk.

BACKGROUND

In manufacturing processes of integrated circuits (ICs), unfinished orfinished circuit components are inspected to ensure that they aremanufactured according to design and are free of defects. Inspectionsystems utilizing optical microscopes or charged particle (e.g.,electron) beam microscopes, such as a scanning electron microscope (SEM)can be employed. As the physical sizes of IC components continue toshrink, accuracy and yield in defect detection become more important.Although multiple electron beams may be used to increase the throughput,the limitation in fidelity of imaging signals received bycharged-particle detectors may limit the imaging resolution desired forreliable defect detection and analysis rendering the inspection toolsinadequate for their desired purpose.

SUMMARY

In some embodiments of the present disclosure, an electro-optical systemof a multi charged-particle beam apparatus is disclosed. Theelectro-optical system may comprise a first pre-limit aperture platecomprising a first aperture configured to block peripheralcharged-particles of a plurality of secondary charged-particle beamsfrom a sample, and a beam-limit aperture array comprising a secondaperture configured to trim the plurality of secondary charged-particlebeams. The electro-optical system may further comprise acharged-particle detector including a plurality of detection elements,wherein a detection element of the plurality of detection elements isassociated with a corresponding trimmed beam of the plurality ofsecondary charged-particle beams.

The distance between the first pre-limit aperture plate and thebeam-limit aperture array may be 5 mm or less. The first pre-limitaperture plate may be positioned upstream or downstream from thebeam-limit aperture array. The electro-optical system may comprise asecond pre-limit aperture plate. The first pre-limit aperture plate maybe positioned upstream from the beam-limit aperture array and the secondpre-limit aperture plate may be positioned downstream from thebeam-limit aperture array. The beam-limit aperture array may comprise aplurality of apertures of different sizes. At least two of the pluralityof apertures may have similar sizes. The plurality of apertures may bearranged in a rectangular, a circular, or a spiral pattern.

The plurality of secondary charged-particle beams may comprise at leastone of secondary electrons or back-scattered electrons generated fromthe sample in response to an interaction between a plurality of primarycharged-particle beams and the sample. The plurality of secondarycharged-particle beams may overlap to create a crossover area on acrossover plane perpendicular to a secondary optical axis of theelectro-optical system. The beam-limit aperture array may be placed onor within a range of positions of the crossover plane and perpendicularto the secondary optical axis. The range of positions of the crossoverplane may be determined based on a landing energy of the plurality ofprimary charged-particle beams on the sample. The beam-limit aperturearray may be movable along the secondary optical axis based on the rangeof positions of the crossover plane.

The second aperture may be centered with the crossover area. The centersof the first and the second apertures may be aligned with the secondaryoptical axis. The beam-limit aperture array may be movable to align anaperture of the plurality of apertures with the crossover area. A planeof the first pre-limit aperture plate may be outside the range ofpositions of the crossover plane and planes of the beam-limit aperturearray and the first pre-limit aperture plate may be within the range ofpositions of the crossover plane.

In another embodiment of the present disclosure, a multicharged-particle beam apparatus is disclosed. The multi charged-particlebeam apparatus may include an electro-optical system for projecting aplurality of secondary charged-particle beams from a sample onto acharged-particle detector. The electro-optical system may comprise afirst pre-limit aperture plate comprising a first aperture configured toblock peripheral charged-particles of the plurality of secondarycharged-particle beams, and a beam-limit aperture array comprising asecond aperture configured to trim the plurality of secondarycharged-particle beams. The charged-particle detector may include aplurality of detection elements, and a detection element of theplurality of detection elements may be associated with a correspondingtrimmed beam of the plurality of secondary charged-particle beams.

In some embodiments of the present disclosure, a method performed by asecondary imaging system to form images of a sample may be disclosed.The method may include generating a plurality of secondarycharged-particle beams from the sample, blocking peripheralcharged-particles of the plurality of secondary charged-particle beamsusing a pre-limit aperture plate, trimming the plurality of secondarycharged-particle beams using an aperture of a beam-limit aperture array;and projecting the plurality of trimmed secondary charged-particle beamsonto a corresponding detection element of a charged-particle detector.

Other advantages of the embodiments of the present disclosure willbecome apparent from the following description taken in conjunction withthe accompanying drawings wherein are set forth, by way of illustrationand example, certain embodiments of the present invention.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a schematic diagram illustrating an exemplary electron beaminspection (EBI) system, consistent with embodiments of the presentdisclosure.

FIG. 2 is a schematic diagram illustrating an exemplary electron beamtool that can be a part of the exemplary electron beam inspection systemof FIG. 1, consistent with embodiments of the present disclosure.

FIG. 3 is a schematic diagram illustrating an exemplary configuration ofa secondary imaging system in a multi-beam apparatus, consistent withembodiments of the present disclosure.

FIG. 4 is a schematic diagram illustrating an exemplary arrangement ofapertures on an aperture array of secondary imaging system of FIG. 3,consistent with embodiments of the present disclosure.

FIG. 5 is a schematic diagram illustrating an exemplary configuration ofa secondary imaging system in a multi-beam apparatus, consistent withembodiments of the present disclosure.

FIGS. 6A-6D are schematic diagrams illustrating exemplary arrangement ofapertures of an aperture array of secondary imaging system of FIG. 5,consistent with embodiments of the present disclosure.

FIG. 7 is a schematic diagram illustrating an exemplary configuration ofa secondary imaging system in a multi-beam apparatus, consistent withembodiments of the present disclosure.

FIG. 8 is a schematic diagram illustrating an exemplary configuration ofa secondary imaging system in a multi-beam apparatus, consistent withembodiments of the present disclosure.

FIG. 9 is a process flowchart representing an exemplary method ofdetecting secondary charged particles from a sample using a secondaryimaging system of FIG. 5, consistent with embodiments of the presentdisclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations. Instead, they are merely examples of apparatuses andmethods consistent with aspects related to the disclosed embodiments asrecited in the appended claims. For example, although some embodimentsare described in the context of utilizing electron beams, the disclosureis not so limited. Other types of charged particle beams may besimilarly applied. Furthermore, other imaging systems may be used, suchas optical imaging, photo detection, x-ray detection, etc.

Electronic devices are constructed of circuits formed on a piece ofsilicon called a substrate. Many circuits may be formed together on thesame piece of silicon and are called integrated circuits or ICs. Thesize of these circuits has decreased dramatically so that many more ofthem can fit on the substrate. For example, an IC chip in a smart phonecan be as small as a thumbnail and yet may include over 2 billiontransistors, the size of each transistor being less than 1/1000th thesize of a human hair.

Making these extremely small ICs is a complex, time-consuming, andexpensive process, often involving hundreds of individual steps. Errorsin even one step have the potential to result in defects in the finishedIC rendering it useless. Thus, one goal of the manufacturing process isto avoid such defects to maximize the number of functional ICs made inthe process, that is, to improve the overall yield of the process.

One component of improving yield is monitoring the chip making processto ensure that it is producing a sufficient number of functionalintegrated circuits. One way to monitor the process is to inspect thechip circuit structures at various stages of their formation. Inspectioncan be carried out using a scanning electron microscope (SEM). An SEMcan be used to image these extremely small structures, in effect, takinga “picture” of the structures. The image can be used to determine if thestructure was formed properly and also if it was formed in the properlocation. If the structure is defective, then the process can beadjusted so the defect is less likely to recur.

Although a multiple charged-beam particle imaging system, such as amulti-beam SEM, may be useful in increasing the wafer inspectionthroughput, the imaging resolution of multi-beam SEM may be limited bythe quality of the imaging signals received and detected by thesecondary electron detection system. A secondary charged-particle beamsuch as an electron beam, generated by the interaction of primarybeamlets on sample surface, may comprise secondary electrons with alarge energy spread of ˜50 eV and a large emission angle range of ˜90°with respect to a normal of the sample surface. Such defocused electronbeams may have a large incidence spot on a secondary electron detector.In conventional multi-beam SEMs, the defocused electron beam may beincident upon multiple detection elements of the secondary electrondetector. In other words, each of the multiple detection elements mayreceive secondary electrons from a corresponding secondary electron beamand other adjacent beams. Consequently, the imaging signal of onedetection element may comprise a main component originating from thecorresponding secondary electron beam and a crosstalk componentoriginating from adjacent electron beams. The crosstalk component, amongother things, may deteriorate the fidelity of the imaging signal.Therefore, it is desirable to minimize crosstalk between multipledetection elements to enhance the imaging resolution.

To mitigate the occurrence of crosstalk, an aperture mechanism may beemployed in a secondary imaging system to block off peripheral secondaryelectrons. However, if the secondary electron beam radius is larger thanthe distance between two apertures of the aperture mechanism, some ofthe electrons of the secondary electron beam may escape through theadjacent aperture, resulting in crosstalk between detection elements ofthe secondary electron detector.

To accommodate the large radii of the secondary electron beams, theapertures may be placed farther apart, and the aperture mechanism may bemoved in the x and y axes relative to the optical axis (z axis) of thesecondary imaging system. However, the movement of aperture mechanism inthe x and y axes may be limited due to space restrictions within thesecondary imaging system, and therefore may limit the number ofapertures within the aperture array. One of the several problems thatmay be encountered is that the aperture mechanism may not be capable ofmitigating crosstalk while allowing maximum number of secondaryelectrons to pass through for enhanced imaging resolution.

In some embodiments of the present disclosure, a multi-beam apparatusmay include an electro-optical system for projecting a plurality ofsecondary electrons from a sample onto a charged-particle detector. Theelectro-optical system may comprise a pre-limit aperture plateconfigured to block peripheral secondary electrons and a beam-limitaperture array comprising an aperture configured to trim the pluralityof secondary electrons. The pre-limit aperture plate may be positionedupstream from the beam-limit aperture array and may prevent peripheralsecondary electrons from irradiating unintended apertures of beam-limitaperture array, and the beam-limit aperture array may further preventperipheral secondary electrons from irradiating detection elements of anelectron detector. The combination of a pre-limit aperture plate and abeam-limit aperture array may mitigate the crosstalk between detectionelements, thus enhancing imaging resolution.

Relative dimensions of components in drawings may be exaggerated forclarity. Within the following description of drawings, the same or likereference numbers refer to the same or like components or entities, andonly the differences with respect to the individual embodiments aredescribed. As used herein, unless specifically stated otherwise, theterm “or” encompasses all possible combinations, except whereinfeasible. For example, if it is stated that a component may include Aor B, then, unless specifically stated otherwise or infeasible, thecomponent may include A, or B, or A and B. As a second example, if it isstated that a component may include A, B, or C, then, unlessspecifically stated otherwise or infeasible, the component may includeA, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

Reference is now made to FIG. 1, which illustrates an exemplary electronbeam inspection (EBI) system 100 consistent with embodiments of thepresent disclosure. As shown in FIG. 1, charged particle beam inspectionsystem 100 includes a main chamber 10, a load-lock chamber 20, anelectron beam tool 40, and an equipment front end module (EFEM) 30.Electron beam tool 40 is located within main chamber 10. While thedescription and drawings are directed to an electron beam, it isappreciated that the embodiments are not used to limit the presentdisclosure to specific charged particles.

EFEM 30 includes a first loading port 30 a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30 b receive wafer front opening unified pods(FOUPs) that contain wafers (e.g., semiconductor wafers or wafers madeof other material(s)) or samples to be inspected (wafers and samples arecollectively referred to as “wafers” hereafter). One or more robot arms(not shown) in EFEM 30 transport the wafers to load-lock chamber 20.

Load-lock chamber 20 is connected to a load/lock vacuum pump system (notshown), which removes gas molecules in load-lock chamber 20 to reach afirst pressure below the atmospheric pressure. After reaching the firstpressure, one or more robot arms (not shown) transport the wafer fromload-lock chamber 20 to main chamber 10. Main chamber 10 is connected toa main chamber vacuum pump system (not shown), which removes gasmolecules in main chamber 10 to reach a second pressure below the firstpressure. After reaching the second pressure, the wafer is subject toinspection by electron beam tool 40. In some embodiments, electron beamtool 40 may comprise a single-beam inspection tool. In otherembodiments, electron beam tool 40 may comprise a multi-beam inspectiontool.

Controller 50 may be electronically connected to electron beam tool 40and may be electronically connected to other components as well.Controller 50 may be a computer configured to execute various controlsof charged particle beam inspection system 100. Controller 50 may alsoinclude processing circuitry configured to execute various signal andimage processing functions. While controller 50 is shown in FIG. 1 asbeing outside of the structure that includes main chamber 10, load-lockchamber 20, and EFEM 30, it is appreciated that controller 50 can bepart of the structure.

While the present disclosure provides examples of main chamber 10housing an electron beam inspection system, it should be noted thataspects of the disclosure in their broadest sense, are not limited to achamber housing an electron beam inspection system. Rather, it isappreciated that the foregoing principles may be applied to otherchambers as well.

Reference is now made to FIG. 2, which illustrates a schematic diagramillustrating an exemplary electron beam tool 40 that can be a part ofthe exemplary charged particle beam inspection system 100 of FIG. 1,consistent with embodiments of the present disclosure. An electron beamtool 40 (also referred to herein as apparatus 40) comprises an electronsource 101, a gun aperture plate 171 with a gun aperture 103, apre-beamlet forming mechanism 172, a condenser lens 110, a sourceconversion unit 120, a primary projection optical system 130, a samplestage (not shown in FIG. 2), a secondary imaging system 150, and anelectron detection device 140. Primary projection optical system 130 cancomprise an objective lens 131. Electron detection device 140 cancomprise a plurality of detection elements 140_1, 140_2, and 140_3. Beamseparator 160 and deflection scanning unit 132 can be placed insideprimary projection optical system 130. It may be appreciated that othercommonly known components of apparatus 40 may be added/omitted asappropriate.

Electron source 101, gun aperture plate 171, condenser lens 110, sourceconversion unit 120, beam separator 160, deflection scanning unit 132,and primary projection optical system 130 can be aligned with a primaryoptical axis 100_1 of apparatus 100. Secondary imaging system 150 andelectron detection device 140 can be aligned with a secondary opticalaxis 150_1 of apparatus 40.

Electron source 101 can comprise a cathode, an extractor or an anode,wherein primary electrons can be emitted from the cathode and extractedor accelerated to form a primary electron beam 102 that forms acrossover (virtual or real) 101 s. Primary electron beam 102 can bevisualized as being emitted from crossover 101 s.

Source conversion unit 120 may comprise an image-forming element array(not shown in FIG. 2), an aberration compensator array (not shown), abeam-limit aperture array (not shown), and a pre-bending micro-deflectorarray (not shown). The image-forming element array can comprise aplurality of micro-deflectors or micro-lenses to form a plurality ofparallel images (virtual or real) of crossover 101 s with a plurality ofbeamlets of primary electron beam 102. FIG. 2 shows three beamlets102_1, 102_2, and 102_3 as an example, and it is appreciated that thesource conversion unit 120 can handle any number of beamlets.

In some embodiments, source conversion unit 120 may be provided withbeam-limit aperture array and image-forming element array (both are notshown). The beam-limit aperture array may comprise beam-limit apertures.It is appreciated that any number of apertures may be used, asappropriate. Beam-limit apertures may be configured to limit sizes ofbeamlets 102_1, 102_2, and 102_3 of primary-electron beam 102. Theimage-forming element array may comprise image-forming deflectors (notshown) configured to deflect beamlets 102_1, 102_2, and 102_3 by varyingangles towards primary optical axis 100_1. In some embodiments,deflectors further away from primary optical axis 100_1 may deflectbeamlets to a greater extent. Furthermore, image-forming element arraymay comprise multiple layers (not illustrated), and deflectors may beprovided in separate layers. Deflectors may be configured to beindividually controlled independent from one another. In someembodiments, a deflector may be controlled to adjust a pitch of probespots (e.g., 102_15, 102_2S, and 102_3S) formed on a surface of sample1. As referred to herein, pitch of the probe spots may be defined as thedistance between two immediately adjacent probe spots on the surface ofsample 1.

A centrally located deflector of image-forming element array may bealigned with primary optical axis 100_1 of electron beam tool 40. Thus,in some embodiments, a central deflector may be configured to maintainthe trajectory of beamlet 102_1 to be straight. In some embodiments, thecentral deflector may be omitted. However, in some embodiments, primaryelectron source 101 may not necessarily be aligned with the center ofsource conversion unit 120. Furthermore, it is appreciated that whileFIG. 2 shows a side view of apparatus 40 where beamlet 102_1 is onprimary optical axis 100_1, beamlet 102_1 may be off primary opticalaxis 100_1 when viewed from a different side. That is, in someembodiments, all of beamlets 102_1, 102_2, and 102_3 may be off-axis. Anoff-axis component may be offset relative to primary optical axis 100_1.

The deflection angles of the deflected beamlets may be set based on oneor more criteria. In some embodiments, deflectors may deflect off-axisbeamlets radially outward or away (not illustrated) from primary opticalaxis 100_1. In some embodiments, deflectors may be configured to deflectoff-axis beamlets radially inward or towards primary optical axis 100_1.Deflection angles of the beamlets may be set so that beamlets 102_1,102_2, and 102_3 land perpendicularly on sample 1. Off-axis aberrationsof images due to lenses, such as objective lens 131, may be reduced byadjusting paths of the beamlets passing through the lenses. Therefore,deflection angles of off-axis beamlets 102_2 and 102_3 may be set sothat probe spots 102_2S and 102_3S have small aberrations. Beamlets maybe deflected so as to pass through or close to the front focal point ofobjective lens 131 to decrease aberrations of off-axis probe spots102_2S and 102_3S. In some embodiments, deflectors may be set to makebeamlets 102_1, 102_2, and 102_3 land perpendicularly on sample 1 whileprobe spots 102_1S, 102_2S, and 102_3S have small aberrations.

Condenser lens 110 is configured to focus primary electron beam 102. Theelectric currents of beamlets 102_1, 102_2, and 102_3 downstream ofsource conversion unit 120 can be varied by adjusting the focusing powerof condenser lens 110 or by changing the radial sizes of thecorresponding beam-limit apertures within the beam-limit aperture array.The electric currents may be changed by both, altering the radial sizesof beam-limit apertures and the focusing power of condenser lens 110.Condenser lens 110 may be an adjustable condenser lens that may beconfigured so that the position of its first principle plane is movable.The adjustable condenser lens may be configured to be magnetic, whichmay result in off-axis beamlets 102_2 and 102_3 illuminating sourceconversion unit 120 with rotation angles. The rotation angles may changewith the focusing power or the position of the first principal plane ofthe adjustable condenser lens. Accordingly, condenser lens 110 may be ananti-rotation condenser lens that may be configured to keep the rotationangles unchanged while the focusing power of condenser lens 110 ischanged. In some embodiments, condenser lens 110 may be an adjustableanti-rotation condenser lens, in which the rotation angles do not changewhen the focusing power and the position of the first principal plane ofcondenser lens 110 are varied.

Electron beam tool 40 may comprise pre-beamlet forming mechanism 172. Insome embodiments, electron source 101 may be configured to emit primaryelectrons and form a primary electron beam 102. In some embodiments, gunaperture plate 171 may be configured to block off peripheral electronsof primary electron beam 102 to reduce the Coulomb effect. In someembodiments, pre-beamlet-forming mechanism 172 further cuts theperipheral electrons of primary electron beam 102 to further reduce theCoulomb effect. Primary-electron beam 102 may be trimmed into threeprimary electron beamlets 102_1, 102_2, and 102_3 (or any other numberof beamlets) after passing through pre-beamlet forming mechanism 172.Electron source 101, gun aperture plate 171, pre-beamlet formingmechanism 172, and condenser lens 110 may be aligned with a primaryoptical axis 100_1 of electron beam tool 40.

Pre-beamlet forming mechanism 172 may comprise a Coulomb aperture array.A center aperture, also referred to herein as the on-axis aperture, ofpre-beamlet-forming mechanism 172 and a central deflector of sourceconversion unit 120 may be aligned with primary optical axis 100_1 ofelectron beam tool 40. Pre-beamlet-forming mechanism 172 may be providedwith a plurality of pre-trimming apertures (e.g., a Coulomb aperturearray). In FIG. 2, the three beamlets 102_1, 102_2 and 102_3 aregenerated when primary electron beam 102 passes through the threepre-trimming apertures, and much of the remaining part of primaryelectron beam 102 is cut off. That is, pre-beamlet-forming mechanism 172may trim much or most of the electrons from primary electron beam 102that do not form the three beamlets 102_1, 102_2 and 102_3.Pre-beamlet-forming mechanism 172 may cut off electrons that willultimately not be used to form probe spots 102_1S, 102_2S and 102_3Sbefore primary electron beam 102 enters source conversion unit 120. Insome embodiments, a gun aperture plate 171 may be provided close toelectron source 101 to cut off electrons at an early stage, whilepre-beamlet forming mechanism 172 may also be provided to further cutoff electrons around a plurality of beamlets. Although FIG. 2demonstrates three apertures of pre-beamlet forming mechanism 172, it isappreciated that there may be any number of apertures, as appropriate.

In some embodiments, pre-beamlet forming mechanism 172 may be placedbelow condenser lens 110. Placing pre-beamlet forming mechanism 172closer to electron source 101 may more effectively reduce the Coulombeffect. In some embodiments, gun aperture plate 171 may be omitted whenpre-beamlet forming mechanism 172 is able to be located sufficientlyclose to source 101 while still being manufacturable.

Objective lens 131 may be configured to focus beamlets 102_1, 102_2, and102_3 onto a sample 1 for inspection and can form three probe spots102_1 s, 102_2 s, and 102_3 s on surface of sample 1. Gun aperture plate171 can block off peripheral electrons of primary electron beam 102 notin use to reduce Coulomb interaction effects. Coulomb interactioneffects can enlarge the size of each of probe spots 102_1 s, 102_2 s,and 102_3 s, and therefore deteriorate inspection resolution.

Beam separator 160 may be a beam separator of Wien filter typecomprising an electrostatic deflector generating an electrostatic dipolefield E1 and a magnetic dipole field B1 (both of which are not shown inFIG. 2). If they are applied, the force exerted by electrostatic dipolefield E1 on an electron of beamlets 102_1, 102_2, and 102_3 is equal inmagnitude and opposite in direction to the force exerted on the electronby magnetic dipole field B1. Beamlets 102_1, 102_2, and 102_3 cantherefore pass straight through beam separator 160 with zero deflectionangles.

Deflection scanning unit 132 can deflect beamlets 102_1, 102_2, and102_3 to scan probe spots 102_1 s, 102_2 s, and 102_3 s over three smallscanned areas in a section of the surface of sample 1. In response toincidence of beamlets 102_1, 102_2, and 102_3 at probe spots 102_1 s,102_2 s, and 102_3 s, three secondary electron beams 102_1 se, 102_2 se,and 102_3 se may be emitted from sample 1. Each of secondary electronbeams 102_1 se, 102_2 se, and 102_3 se can comprise electrons with adistribution of energies including secondary electrons (energies <50 eV)and backscattered electrons (energies between 50 eV and landing energiesof beamlets 102_1, 102_2, and 102_3). Beam separator 160 can directsecondary electron beams 102_1 se, 102_2 se, and 102_3 se towardssecondary imaging system 150. Secondary imaging system 150 can focussecondary electron beams 102_1 se, 102_2 se, and 102_3 se onto detectionelements 140_1, 140_2, and 140_3 of electron detection device 140.Detection elements 140_1, 140_2, and 140_3 can detect correspondingsecondary electron beams 102_1 se, 102_2 se, and 102_3 se and generatecorresponding signals used to construct images of the correspondingscanned areas of sample 1.

In FIG. 2, three secondary electron beams 102_1 se, 102_2 se, and 102_3se respectively generated by three probe spots 102_1S, 102_2S, and102_3S, travel upward towards electron source 101 along primary opticalaxis 100_1, pass through objective lens 131 and deflection scanning unit132 in succession. The three secondary electron beams 102_1 se, 102_2 seand 102_3 se are diverted by beam separator 160 (such as a Wien Filter)to enter secondary imaging system 150 along secondary optical axis 150_1thereof. Secondary imaging system 150 focuses the three secondaryelectron beams 102_1 se˜102_3 se onto electron detection device 140which comprises three detection elements 140_1, 140_2, and 140_3.Therefore, electron detection device 140 can simultaneously generate theimages of the three scanned regions scanned by the three probe spots102_15, 102_2S and 102_3S, respectively. In some embodiments, electrondetection device 140 and secondary imaging system 150 form one detectionunit (not shown). In some embodiments, the electron optics elements onthe paths of secondary electron beams such as, but not limited to,objective lens 131, deflection scanning unit 132, beam separator 160,secondary imaging system 150 and electron detection device 140, may formone detection system.

In some embodiments, controller 50 may comprise an image processingsystem that includes an image acquirer (not shown) and a storage (notshown). The image acquirer may comprise one or more processors. Forexample, the image acquirer may comprise a computer, server, mainframehost, terminals, personal computer, any kind of mobile computingdevices, and the like, or a combination thereof. The image acquirer maybe communicatively coupled to electron detection device 140 of apparatus40 through a medium such as an electrical conductor, optical fibercable, portable storage media, IR, Bluetooth, internet, wirelessnetwork, wireless radio, among others, or a combination thereof. In someembodiments, the image acquirer may receive a signal from electrondetection device 140 and may construct an image. The image acquirer maythus acquire images of sample 1. The image acquirer may also performvarious post-processing functions, such as generating contours,superimposing indicators on an acquired image, and the like. The imageacquirer may be configured to perform adjustments of brightness andcontrast, etc. of acquired images. In some embodiments, the storage maybe a storage medium such as a hard disk, flash drive, cloud storage,random access memory (RAM), other types of computer readable memory, andthe like. The storage may be coupled with the image acquirer and may beused for saving scanned raw image data as original images, andpost-processed images.

In some embodiments, the image acquirer may acquire one or more imagesof a sample based on an imaging signal received from electron detectiondevice 140. An imaging signal may correspond to a scanning operation forconducting charged particle imaging. An acquired image may be a singleimage comprising a plurality of imaging areas. The single image may bestored in the storage. The single image may be an original image thatmay be divided into a plurality of regions. Each of the regions maycomprise one imaging area containing a feature of sample 1. The acquiredimages may comprise multiple images of a single imaging area of sample 1sampled multiple times over a time sequence. The multiple images may bestored in the storage. In some embodiments, controller 50 may beconfigured to perform image processing steps with the multiple images ofthe same location of sample 1.

In some embodiments, controller 50 may include measurement circuitries(e.g., analog-to-digital converters) to obtain a distribution of thedetected secondary electrons. The electron distribution data collectedduring a detection time window, in combination with corresponding scanpath data of each of primary beamlets 102_1, 102_2, and 102_3 incidenton the wafer surface, can be used to reconstruct images of the waferstructures under inspection. The reconstructed images can be used toreveal various features of the internal or external structures of sample1, and thereby can be used to reveal any defects that may exist in thewafer.

In some embodiments, controller 50 may control a motorized stage (notshown) to move sample 1 during inspection. In some embodiments,controller 50 may enable the motorized stage to move sample 1 in adirection continuously at a constant speed. In other embodiments,controller 50 may enable the motorized stage to change the speed of themovement of sample 1 over time depending on the steps of scanningprocess. In some embodiments, controller 50 may adjust a configurationof primary projection optical system 130 or secondary imaging system 150based on images of secondary electron beams 102_1 se, 102_2 se, and102_3 se.

Although FIG. 2 shows that electron beam tool 40 uses three primaryelectron beams, it is appreciated that electron beam tool 40 may use twoor more number of primary electron beams. The present disclosure doesnot limit the number of primary electron beams used in apparatus 40.

Reference is now made to FIG. 3, which is a schematic diagram of anexemplary configuration of a secondary imaging system in a multi-beamapparatus, consistent with embodiments of the present disclosure. It isappreciated that secondary imaging system 150 may be part ofcharged-particle beam inspection system (e.g., electron beam inspectionsystem 100 of FIG. 1).

In some embodiments, secondary imaging system 150 will be shown anddescribed together with the entire detection system, as illustrated inFIG. 3. With reference to FIG. 3, only three secondary electron beamswith respect to three probe spots are shown, although there may be anynumber of secondary electron beams. Within the entire detection system,starting from sample 1, the first part is along primary optical axis100_1 and the second part is along secondary optical axis 150_1. Forillustrative purposes only and bearing no resemblance to the actualconfiguration, the first part is rotated to be along secondary opticalaxis 150_1 such that the entire detection system can be shown along onestraight optical axis.

In some embodiments, as illustrated in FIG. 3, secondary imaging system150 may include zoom lens 151, projection lens 152, secondary beam-limitaperture array 155, and anti-scanning deflection unit (not shown), allaligned with secondary optical axis 150_1. Detection elements 140_1,140_2, and 140_3 of electron detection device 140 may be placed on adetection plane SP3, normal to secondary optical axis 150_1. Zoom lens151, projection lens 152, and objective lens 131 together project thesurface of sample 1 onto a detection plane SP3, i.e. focus the secondaryelectron beams 102_1 se-102_3 se to form secondary-electron spots ondetection elements 140_1, 140_2 and 140_3, respectively, when deflectionscanning unit 132 is off.

In some embodiments, zoom lens 151 may comprise two electrostatic lenses151_11 and 151_12. The image plane of zoom lens 151 may be at a transferplane SP2, as illustrated in FIG. 3. Projection lens 152 may compriseone electrostatic lens and one magnetic lens (both are not shown), andthe image plane thereof may be at detection plane SP3. The first imagingmagnification M1 from the surface of sample 1 to transfer plane SP2 maybe realized by objective lens 131 and zoom lens 151, whereas the secondimaging magnification M2 from transfer plane SP2 to detection plane SP3may be realized by projection lens 152, and the total imagingmagnification M from the surface of sample 1 to detection plane SP3 maybe determined based on M1 and M2. Specifically, the total imagingmagnification M may be based on M1*M2.

In some embodiments, zoom lens 151 may be configured to perform the zoomfunction. By adjusting the focusing power of the two electrostaticlenses 151_11 and 151_12, the first imaging magnification M1 can bevaried to achieve the desired value of the total imaging magnificationM. Projection lens 152 may be configured to perform the anti-rotatingfunction. By adjusting the magnetic field of the magnetic lens and thefocusing power of the electrostatic lens, the total image rotation ondetection plane SP3 and the second imaging magnification M2 can remainthe same. The anti-scanning deflection unit (not shown) may beconfigured to perform the anti-scanning function. By synchronouslydeflecting the secondary electron beams with deflection scanning unit132, the displacements of three secondary electron spots on thedetection plane SP3 can be substantially canceled. Consequently, thecorresponding relationship between the plurality of probe spots and theplurality of detection elements can always be kept. To reduce theadditional aberrations of zoom lens 151 and projection lens 152, whichcome from the deflection of the secondary electron beams generated bydeflection scanning unit 132, the anti-scanning deflection unit isbetter placed before zoom lens 151 and hence the secondary electronbeams will pass through zoom lens 151 and projection lens 152 in a wayas if deflection scanning unit 132 is off. However, in this case, zoomlens 151 may be placed far away from beam separator 160 and,consequently may generate large initial aberrations.

As is commonly known in the art, the emission of secondary electronsobeys Lambert's law and has a large energy spread. While the energy of asecondary electron may be up to 50 eV, most have an energy ofapproximately 5 eV, depending on the sample material, among otherthings. The landing energy of the primary electron beamlets, such as theenergy of beamlet 102_1 as it lands on the sample, may be in the rangeof 0.1 keV to 5 keV. The landing energy may be adjusted by varyingeither or both of the bias of primary electron source 101 or the bias ofsample 1. Therefore, the excitation of objective lens 131 may beadjusted to provide the corresponding focusing power for the threebeamlets. Further, for reduced aberrations, objective lens 131 may be amagnetic or an electromagnetic compound lens configured to rotate thebeamlets and affect the landing energy. Because the size, the position,or the magnification of the secondary electron spots formed by thesecondary electron beams 102_1 se, 102_2 se, and 102_3 se on detectionelements 140_1, 140_2, and 140_3 may vary, the secondary electron spotsmay partially enter a detection element adjacent to the correspondingdetection element. The secondary electrons detected by the adjacentdetection elements may generate image overlaps, for example, causingdeterioration of image resolution. The image signal from one detectionelement may include information from more than one scanned region ofsample 1, resulting in loss of resolution due to crosstalk.

Crosstalk in multi-beam SEMs such as EBI system 100 may be mitigated byusing a secondary beam-limit aperture array 155 to cut off theperipheral secondary electrons of the secondary electron beam such as102_1 se, for example Secondary beam-limit aperture array 155 maycomprise a plurality of apertures. Although FIG. 3 illustrates only twoapertures, 155_1 and 155_2, any number of apertures may be used, asappropriate. For example, secondary beam-limit aperture array 155illustrated in FIG. 4 (described later) comprises six apertures, 155_1,155_2, 155_3, 155_4, 155_5, and 155_6.

In general, when the size of an aperture (e.g., aperture 155_1 of FIG.3) of secondary beam-limit aperture 155 increases, the overallcollection efficiency of a secondary electron detector may increase.However, the difference in collection efficiencies of each detectionelement of the detector may also increase, and the crosstalk amongdetection elements 140_1, 140_2, and 140_3 may increase as well.Although the increase in overall collection efficiency of electrondetection device 140 increases the throughput, however, the differencein collection efficiencies of each detection element 140_1, 140_2, and140_3 may cause the grey levels of images formed by secondary electronbeams 102_1 se-102_3 se to differ more. One or more additional processesmay be performed to eliminate the inspection errors due to thedifference of grey levels, thereby decreasing inspection throughput anddeteriorating resolution of the MBI apparatus. When the crosstalk amongsecondary electron beams 102_1 se-102_3 se increases, the images formedby secondary electron beams 102_1 se-102_3 se may be degraded. That is,large crosstalk deteriorates inspection resolution of the MBI apparatus.

In some embodiments, each of the apertures of secondary beam-limitaperture array 155 may have uniform or non-uniform size, shape, orcross-section. The smaller the radial size of an aperture, the lower thecollection efficiencies and the crosstalk of the imaging signals ofdetection elements 140_1, 140_2, and 140_3 will generally be. Therefore,the radial size of the apertures may depend on the application or thedesired outcome.

In some embodiments in which objective lens 131 functions in anon-magnetic immersion mode, the angular velocity of the emergingsecondary electrons may be zero on the sample surface. In suchembodiments, the chief rays of off-axis secondary electron beams 102_2se and 102_3 se may still be meridional after exiting objective lens 131and may be able to cross secondary optical axis 150_1 of secondaryimaging system 150. Furthermore, the chief rays can cross secondaryoptical axis 150_1 at a same place (if aberrations are not considered)in secondary imaging system 150. As such, secondary electron beams 102_1se-102_3 se may be configured to overlap at a common area of crossingand therefore form a relatively sharp secondary beam crossover. Theplane where the common area of crossing or secondary beam crossover islocated is referred to as a crossing plane or secondary beam crossoverplane.

While FIG. 3 illustrates an exemplary relatively sharp secondary beamcrossover plane formed by fully overlapping secondary beams 102_1se-102_3 se on one crossing plane, it is appreciated that one or more ofthe secondary electron beams may be offset from others on the crossingplane and the secondary beam crossover may not be that sharp, forming arange of secondary beam crossover planes along secondary optical axis150_1. The position of secondary beam crossover plane may depend onlanding energies of primary beamlets or excitations of objective lens131, among other things. In some embodiments, secondary beam-limitaperture array 155 may be positioned on the secondary beam crossoverplane, or in other words, the plane of secondary beam-limit aperturearray 155 may coincide with the secondary beam crossover plane. In someembodiments, the plane of secondary beam limit aperture array 155 may bewithin a moving range of positions of secondary beam crossover plane.Secondary beam-limit aperture array 155 may be moved along secondarybeam crossover plane such that the desired aperture or aperture size maybe used to block off peripheral secondary electrons directed towardselectron detection device 140. In some embodiments, secondary beam-limitaperture array 155 may be placed at an optimal position within the rangeof positions of the secondary beam crossover plane.

Reference is now made to FIG. 4, which illustrates a schematic diagramof exemplary arrangement of apertures on secondary beam-limit aperturearray 155, consistent with embodiments of the present disclosure.Although beam-limit aperture array 155 comprises six apertures ofvarying sizes are illustrated in FIG. 4, any number of apertures may beused.

As shown in FIG. 4, a cross-section 102 se represents a cross-section ofthe overlapping secondary electron beams 102_1 se-102_3 se incident onsecondary beam-limit aperture array 155. In some embodiments, secondaryelectron beams 102_1 se-102_3 se may not overlap at the same crossingplane to form a sharp secondary beam crossover, but instead may beoffset along secondary optical axis 150_1 such that they form a range ofcrossing planes. In such cases, cross-section 102 se of overlappingsecondary electron beams 102_1 se-102_3 se may not be sharp.

In some embodiments, to minimize the occurrence of crosstalk, thedistance between two adjacent apertures such as 155_2 and 155_3 may belarger than the sum of radius R of overlapped secondary electron beam102 se and a radius of the larger one of the two apertures. In someembodiments, secondary beam-limit aperture array 155 may comprise atleast two apertures of equal size, and in such cases, the distancebetween two adjacent apertures such as 155_2 and 155_3 may be largerthan the sum of radius R of cross-section 102 se of overlappingsecondary electron beams 102_1 se-102_3 se and a radius of the eitherone of the two apertures.

In some embodiments, radius R of cross-section 102 se of overlappingsecondary electron beams 102_1 se-102_3 se may depend on landingenergies of primary beamlets or excitations of objective lens 131.Therefore, to accommodate a wide range of radii of overlapping secondaryelectron beams 102_1 se-102_3 se (represented by cross-section 102 se ofFIG. 4), the distance between two adjacent apertures may be determinedbased on the largest value of radius R.

Reference is now made to FIG. 5, which illustrates an exemplaryconfiguration of a secondary imaging system 150 in an electron beam tool500 of a multi-beam apparatus, consistent with embodiments of thepresent disclosure. In comparison to secondary imaging system 150 ofelectron beam tool 40 of FIG. 3, secondary imaging system 150 ofelectron beam tool 500 may include a pre-limit aperture plate 155P.

As illustrated in FIG. 5, pre-limit aperture plate 155P may comprise aplate with an aperture configured to block peripheral electrons whileallowing axial electrons of secondary electron beams 102_1 se-102_3 se.In some embodiments, pre-limit aperture plate 155P may be aligned withsecondary beam-limit aperture array 155 and secondary optical axis 150_1such that it blocks most of the peripheral electrons of off-axissecondary electron beams 102_2 se and 102_3 se. In some embodiments,pre-limit aperture plate 155P may be configured to prevent secondaryelectrons from irradiating unintended apertures of secondary beam-limitaperture array 155.

In some embodiments, pre-limit aperture plate 155P may be positionedupstream from secondary beam-limit aperture array 155, as illustrated inFIG. 5. In the context of this disclosure, “upstream from” refers to theposition of pre-limit aperture plate 155P such that one or moresecondary electron beams 102_1 se-102_3 se may be incident on it beforeirradiating secondary beam-limit aperture array 155.

Pre-limit aperture plate 155P may be placed before secondary beam-limitaperture array 155 such that the aperture of pre-limit aperture plate155P aligns with a desired aperture of secondary beam-limit aperturearray 155. In such cases, the number of peripheral secondary electronsincident on secondary beam-limit aperture array 155 may be reduced orminimized. In some embodiments, pre-limit aperture plate 155P may beconfigured to block peripheral secondary electrons from being incidenton other unintended apertures of secondary beam-limit aperture array155, thus reducing crosstalk. Such a configuration may also enable theapertures of secondary beam-limit aperture array 155 to be positionedcloser to each other, thereby allowing more apertures to be placed in asmaller area on aperture array 155, while mitigating the occurrence ofcrosstalk.

The size and shape of the apertures of secondary beam-limit aperturearray 155 may vary over time based on the duration of usage, materials,inspection parameters, etc. For example, exposure to secondary electronsmay cause contamination and debris formation on the edges of anaperture, thus reducing the effective size and shape of the aperturethrough which secondary electrons may pass. In some embodiments, placingpre-limit aperture plate 155P before secondary beam-limit aperture array155 may reduce or minimize the variation in size and shape of aperturesof secondary beam-limit aperture array 155 by reducing the number ofincident peripheral secondary electrons.

In some embodiments, as illustrated in FIG. 4, apertures 155_1 and 155_2of secondary beam-limit aperture array 155 may be separated from eachother so that secondary electrons passing through pre-limit apertureplate 155P may only pass through one intended aperture of secondarybeam-limit aperture array 155. In some embodiments, pre-limit apertureplate 155P may be configured to block peripheral electrons so that theydo not pass through an aperture adjacent to an intended aperture ofsecondary beam-limit aperture array 155, enabling the apertures ofsecondary beam-limit aperture array 155 to be positioned closer to eachother. This may allow designing aperture arrays comprising more aperturesizes in a smaller area on aperture array 155, while mitigating theoccurrence of crosstalk.

In some embodiments, pre-limit aperture plate 155P may be positioneddownstream from secondary beam-limit aperture array 155 (this embodimentis not illustrated), for example, between secondary beam-limit aperturearray 155 and electron detection device 140. In the context of thisdisclosure, “downstream from” refers to the position of pre-limitaperture plate 155P such that one or more secondary electron beams 102_1se-102_3 se may be incident on it after irradiating secondary beam-limitaperture array 155. Pre-limit aperture plate 155P may be positionedcloser to secondary beam-limit aperture array 155 than electrondetection device 140.

Although not illustrated in FIG. 5, in some embodiments, secondaryimaging system 150 may comprise more than one pre-limit aperture plates155P. For example, a primary pre-limit aperture plate 155P_1 (notillustrated) may be positioned upstream from secondary beam-limitaperture array 155 and a secondary pre-limit aperture plate 155P_2 (notillustrated) may be positioned downstream from secondary beam-limitaperture array 155. In such a configuration, primary pre-limit apertureplate 155P_1 may be configured to block majority of the peripheralsecondary electrons from irradiating unintended apertures of secondarybeam limit aperture array 155, and secondary pre-limit aperture plate155P_2 may be configured to block any stray peripheral secondaryelectrons that may not have been blocked by primary pre-limit apertureplate 155, thus mitigating the occurrence of crosstalk. It isappreciated that other combinations of the number of pre-limit apertureplates and their arrangements are possible as well.

Pre-limit aperture plate 155P and secondary beam-limit aperture array155 may be separated by an optimum distance 155G, as illustrated in FIG.5. While it may be desirable to minimize distance 155G between pre-limitaperture plate 155P and secondary beam-limit aperture array 155 toreduce the possibility of peripheral secondary electrons escaping andilluminating other apertures of secondary beam-limit aperture array 155,distance 155G may be optimized to allow unrestricted movement ofpre-limit aperture plate 155P and secondary beam-limit aperture array155. In some embodiments, the distance between pre-limit aperture plate155P and secondary beam-limit aperture array 155 may be 5 mm. In someembodiments, distance 155G may be determined based on mechanical designconsiderations, available space, manufacturability, and cost-efficiency,among other things. For example, it may be possible, using sometechniques, to reliably and reproducibly achieve a distance of 3 mm to 5mm between pre-limit aperture plate 155P and secondary beam-limitaperture array 155. In some embodiments, distance 155G may be more than5 mm, for example, 10 mm, based on factors including, but not limitedto, space availability, design limitations, cost-efficiency, materials,and intended application.

Reference is now made to FIGS. 6A-6D, which are schematic diagramsillustrating exemplary arrangement of apertures of secondary beam-limitaperture array 155 in secondary imaging system 150 of electron beam tool500 shown in FIG. 5, consistent with embodiments of the presentdisclosure. It is appreciated that the sizes of apertures, size ofsecondary beam-limit aperture array 155, secondary electron beam sizebefore pre-limit aperture plate 155P, and secondary electron beam sizeincident on secondary beam-limit aperture array 155 after passingthrough pre-limit aperture plate 155P are for illustrative purposes onlyand not drawn to scale.

As illustrated in FIGS. 6A-6D, cross-section 102 se represents anoutline of a beam of secondary electrons generated by primary beamlets(e.g., primary beamlets 102_1, 102_2, and 102_3 of FIG. 2) afterinteracting with the sample at probe spots 102_1S, 102_2S, and 102_3S,respectively.

FIG. 6A illustrates a secondary beam-limit aperture array 155Acomprising three apertures, 155_1, 155_2, and 155_3 placed in onedirection such that the geometric centers of all apertures are alignedalong an axis, for example, x axis. Although only three apertures areshown, any number of apertures may be used. In some embodiments, thenumber of apertures in such a configuration may be limited by theallowable size of secondary beam-limit aperture array 155A based onphysical space available in secondary imaging system 150. One of theseveral advantages of using pre-limit aperture plate 155P is that itallows reduction in separation distance between two adjacent aperturesby blocking off all the peripheral secondary electrons from illuminatingapertures of secondary beam-limit aperture array 155A not in use fordetection. In addition, using pre-limit aperture plate 155P may allowreduction in crosstalk.

The reduced separation distance between two adjacent apertures is shownin FIG. 6A. In this context, the separation distance between twoapertures may be referred to as the linear distance between thegeometric centers of the two adjacent apertures. In some embodiments,the separation distance between two apertures across the array may beuniform. For example, separation distance between apertures 155_1 and155_2 may be similar to the separation distance between apertures 155_2and 155_3. In some embodiments, the separation distance may benon-uniform.

In some embodiments, secondary beam-limit aperture array 155A may beconfigured to move along a single axis such as the x axis, to adjust thesize of the aperture through which overlapping secondary electron beammay pass.

As shown in FIG. 6A, secondary beam-limit aperture array 155A maycomprise apertures of different sizes. In some embodiments, apertures155_1, 155_2, and 155_3 of secondary beam-limit aperture array 155A maybe uniform in size, shape, cross-section, and pitch. In someembodiments, the radius of aperture of pre-limit aperture plate 155P maybe slightly larger than the radius of the largest aperture of secondarybeam-limit aperture array 155. Pre-limit aperture plate 155P andsecondary beam-limit aperture array 155 may be aligned such that thegeometric centers of the aperture of pre-limit aperture plate 155P andsecondary beam-limit aperture array 155 are aligned with secondaryoptical axis 150_1.

FIG. 6B illustrates a secondary beam-limit aperture array 155Bcomprising six apertures, 155_1-155_6 placed in a rectangular matrixalong x and y axes. In some embodiments, secondary beam-limit aperturearray 155B may comprise apertures of different sizes. In someembodiments, secondary beam-limit aperture array 155B may comprise atleast two apertures of similar size. Cross-section 102 se_155Prepresents an outline of the beam of secondary electrons (represented bycross-section 102 se) generated by primary beamlets 102_1, 102_2, and102_3 and passing through pre-limit aperture plate 155P. It isappreciated that cross-sections 102 se and 102 se_155P representoutlines of secondary electron beams at different planes along secondaryoptical axis 150_1.

In some embodiments, secondary beam-limit aperture array 155B may beconfigured to move along both, the x and y axes, to adjust the size ofthe aperture through which overlapping secondary electron beam may pass.Although FIGS. 6A and 6B illustrate rectangular secondary beam-limitaperture arrays (e.g., 155A and 155B), other shapes including, but notlimited to, circular, triangular, elliptical, etc. may be used. It isappreciated that the size and the shape of secondary beam-limit aperturearrays may be determined based on physical space available, mechanicaldesign considerations, cost-efficiency, etc.

Reference is now made to FIG. 6C, which illustrates a circular secondarybeam-limit aperture array 155C comprising seven apertures, 155_1-155_7.In some embodiments, secondary beam-limit aperture array 155C maycomprise a center aperture 155_1 and six off-center apertures 155-155_7arranged radially around center aperture 155_1. As illustrated in FIG.6C, off-center apertures may be positioned along a virtual circle 155Rso that the separation distance between the center of center aperture155_1 and the center of each of the off-center apertures 155_2-155_7 maybe uniform. In other words, the separation distance between the centerof center aperture 155_1 and the center of each of the off-centerapertures 155_2-155_7 may be similar or substantially similar to theradius of 155R. In such embodiments, the separation distance, or radiusof virtual circle 155R may be determined based on the radius ofcross-section 102 se_155P and the radius of the largest aperture (e.g.,aperture 155_6 of FIG. 6C) of secondary beam-limit aperture array 155C.In some embodiments, the separation distance, may be larger than the sumof the radius of cross-section 102 se_155P and the radius of the largestaperture 155_6, for example, of secondary beam-limit aperture array155C.

As illustrated in FIG. 6C, secondary beam-limit aperture array 155C maycomprise apertures of different sizes. The center aperture 155_1 may bedifferent in size compared to off-center apertures 155-155_7. In someembodiments, each of the off-center apertures may be of different sizes,randomly arranged in size along the perimeter of virtual circle 155R.

In some embodiments, secondary beam-limit aperture array 155C may beconfigured to move along both, x and y-axes, to adjust the size of theaperture through which overlapping secondary electron beam may pass. Oneof the several advantages of a circular arrangement of apertures insecondary beam-limit aperture array 155C is that a variety of aperturesizes may be accessed with limited movement in x and y axes.

FIG. 6D illustrates a circular secondary beam-limit aperture array 155Dcomprising seven apertures, 155_1-155_7. In comparison to FIG. 6C, twoor more apertures of secondary beam-limit aperture array 155D may besimilar or substantially similar in size. For example, off-centerapertures 155_3 and 155_7 are similar in size. Although FIG. 6Dillustrates two off-center apertures 155_3 and 155_7 having similarsizes, it is appreciated that any two or more apertures may have similarsizes.

In some embodiments, apertures may be contaminated after long periods ofuse, for example due to particles, debris, and gas generated bysecondary electrons incident on secondary beam-limit aperture array155D. The contamination may change the effective size or shape of theaperture, affecting the collection efficiency of detection elements(e.g., 140_1, 140_2, and 140_3 of FIG. 3) of electron detection device140, and therefore impacting the overall throughput and resolution ofimages. The problem of contamination may be exaggerated in smallerapertures such as 155_3, partly because small apertures block a largeportion of the secondary electrons incident on the array compared tolarger apertures. In addition, smaller apertures have smallercircumferences, and therefore, for the same duration of exposure tosecondary electrons, the percentage reduction in effective size of theaperture may be larger compared to larger apertures. One of the severaladvantages of having two or more apertures of similar sizes may bereduction in tool down-time due to maintenance. For example, if one ofthe apertures is contaminated, a second aperture of similar size may beused, allowing uninterrupted tool usage and resultantly improvingoverall throughput of wafer inspection.

Reference is now made to FIG. 7, which illustrates an exemplaryconfiguration 700 of a secondary imaging system 150 in a multi-beamapparatus, consistent with embodiments of the present disclosure. It isappreciated that secondary imaging system 150 may be part ofcharged-particle beam inspection system (e.g., electron beam inspectionsystem 100 of FIG. 1).

As illustrated in FIG. 7, secondary imaging system 150 may comprise amovable secondary beam-limit aperture array 155 configured to move inthree directions along the x, y, and z axes. In some embodiments,secondary beam-limit aperture array 155 may be moved along x and y axesto select a desired aperture size, and along z axis to match theposition of secondary beam crossover plane, while pre-limit apertureplate 155P is fixed in its position. The x and y axes may beperpendicular to secondary optical axis 150_1 and the z axis may beparallel to secondary optical axis 150_1.

In some embodiments, the position of secondary beam crossover planealong secondary optical axis 150_1 may be determined based at least onfactors including, but not limited to, landing energies of primarybeamlets, and excitation of objective lens 131. In some embodiments, thecorresponding positions of secondary beam crossover plane for a range oflanding energies of primary beamlets may be determined based onsimulations and data modeling algorithms.

Based on the landing energies of the primary beamlets, the secondaryelectron beams may overlap at different planes perpendicular tosecondary optical axis 150_1, thus forming a range of secondary beamcrossover plane positions for a corresponding range of landing energies.In some embodiments, the crossover plane positions may be determinedbased on a simulation of landing energies, and therefore, the range ofsecondary beam crossover plane positions may be determined based on therange of landing energies used in the system. For example, a user or thesystem may theoretically determine, based on simulation such as analgorithm, the coordinates of the secondary beam crossover plane for agiven value of landing energies of the primary beamlets.

In some embodiments, if the position of secondary beam-limit aperturearray 155 is adjusted along the z axis to align with the position of thesecondary beam crossover plane, then the position of pre-limit apertureplate 155P may fall out of the range of secondary beam crossover planepositions. This may increase the size of secondary electron beams (e.g.,represented as 102 se_155P in FIGS. 6A-6D) incident on secondarybeam-limit aperture array 155. Although a large beam size incident onsecondary beam-limit aperture array 155 may enhance the number ofsecondary electrons incident on detector elements of electron detectiondevice, it may increase the possibility of occurrence of crosstalk aswell.

Reference is now made to FIG. 8, which illustrates an exemplaryconfiguration 800 of a secondary imaging system 150 in a multi-beamapparatus, consistent with embodiments of the present disclosure. It isappreciated that secondary imaging system 150 may be part ofcharged-particle beam inspection system (e.g., electron beam inspectionsystem 100 of FIG. 1).

As illustrated in FIG. 8, secondary imaging system 150 may comprise amovable pre-limit aperture plate 155P and a movable secondary beam-limitaperture array 155. Movable pre-limit aperture plate 155P may beconfigured to independently move in three directions along the x, y andz axes. In some embodiments, movable pre-limit aperture plate 155P maybe configured to move along with movable secondary beam-limit aperturearray 155 such that the distance 155G is maintained.

In some embodiments, movable pre-limit aperture plate 155P and secondarybeam-limit aperture array 155 may be moved together such that theposition of secondary beam-limit aperture array 155 aligns with theposition of the secondary beam crossover plane. In such a scenario,based on distance 155G and the range of crossover positions, pre-limitaperture plate 155P and secondary beam-limit aperture array 155 may bothbe placed within the range of crossover positions. In some embodiments,it may be desirable to have pre-limit aperture plate 155P and secondarybeam-limit aperture array 155 within the range of crossover positionsalong secondary optical axis 150_1 to effectively block peripheralelectrons and reduce the possibility of crosstalk.

Reference is now made to FIG. 9, which illustrates a process flowchartrepresenting an exemplary method 900 performed by a secondary imagingsystem (e.g., secondary imaging system 150 of FIG. 3) to form images ofa sample, consistent with embodiments of the present disclosure. Method900 may be performed by controller 50 of EBI system 100, as shown inFIG. 1, for example Controller 50 may be programmed to implement one ormore steps of method 900. For example, controller 50 may instruct amodule of a charged particle beam apparatus to activate acharged-particle source to generate charged particle beams, which uponinteraction with the sample may generate secondary charged-particlebeams.

In step 910, multiple secondary electron beams (e.g., 102_1 se, 102_2se, 102_3 se of FIG. 3) may be generated from the sample (e.g., sample 1of FIG. 2) upon interaction of primary beamlets (e.g., 102_1, 102_2, and102_3 of FIG. 2) with probe spots (e.g., 102_15, 102_2S, and 102_3S ofFIG. 2) of the sample. In some embodiments, the number of secondaryelectron beams generated may be equal to the number of primary beamletsincident on the sample. The three secondary electron beams 102_1 se,102_2 se and 102_3 se may be diverted by a beam separator such as a WienFilter (e.g., beam separator 160 of FIG. 2) to enter secondary imagingsystem 150 along secondary optical axis 150_1 thereof.

Step 910 may further include directing secondary electron beams 102_1se-102_3 se such that they overlap at a common area of crossing alongthe secondary optical axis and therefore form a secondary beam crossover(e.g., such as a relatively sharp secondary beam crossover). The planewhere the common area of crossing or secondary beam crossover is locatedis referred to as a crossing plane or secondary beam crossover plane. Abeam-limiting aperture array (e.g., secondary beam-limit aperture array155 of FIG. 3) may be positioned at or near the secondary beam crossoverplane. In some embodiments, a pre-limit aperture plate (e.g., pre-limitaperture plate 155P of FIG. 3) may be placed before the secondarybeam-limit aperture array.

In step 920, peripheral secondary electrons of secondary electron beamsmay be blocked using the pre-limit aperture plate before illuminatingthe secondary beam-limit aperture array. Pre-limit aperture plate maycomprise a plate with an aperture configured to block peripheralelectrons while allowing axial electrons of secondary electron beams. Insome embodiments, pre-limit aperture plate may be aligned with secondarybeam-limit aperture array and secondary optical axis such that it blocksmost of the peripheral electrons of off-axis secondary electron beams.

In some embodiments, pre-limit aperture plate and secondary beam-limitaperture array may be separated by an optimum distance (e.g., distance155G as illustrated in FIG. 5). While it may be desirable to minimizethe distance between pre-limit aperture plate and secondary beam-limitaperture array to reduce the possibility of peripheral secondaryelectrons escaping and irradiating other unintended apertures ofsecondary beam-limit aperture array, it may be optimized to allowunrestricted movement of pre-limit aperture plate and secondarybeam-limit aperture array. In some embodiments, the distance betweenpre-limit aperture plate and secondary beam-limit aperture array may be5 mm. In some embodiments, distance may be determined based onmechanical design considerations, available space, manufacturability,and cost-efficiency, among other things. For example, it may bepossible, using some techniques, to reliably and reproducibly achieve adistance of 3 mm to 5 mm between pre-limit aperture plate 155P andsecondary beam-limit aperture array 155. In some embodiments, thedistance may be more than 5 mm, for example, 10 mm, based on factorsincluding, but not limited to, space availability, design limitations,cost-efficiency, materials, and intended application.

In step 930, the secondary electron beams may be further trimmed usingthe beam-limit aperture array. The secondary beam-limit aperture arraymay be placed at or within a moving range of positions of the secondarybeam crossover plane. The secondary beam-limit aperture array may bemoved along secondary beam crossover plane such that the desiredaperture or aperture size may be used to block off peripheral secondaryelectrons directed towards charged-particle detection elements (e.g.,electron detection device 140 of FIG. 3). The position of secondary beamcrossover plane may depend on landing energy of primary beamlets orexcitations of objective lens (e.g., objective lens 131 of FIG. 2),among other things. Secondary beam-limit aperture array may be placed atan optimal position within a secondary beam crossover plane movingrange.

In some embodiments, the order of steps 920 and 930 may beinterchangeable. For example, the pre-limit aperture plate may be placedupstream from or downstream from the secondary beam limit aperture arraysuch that the secondary electron beams 102_1 se-102_3 se may be incidenton it before or after irradiating the secondary beam-limit aperturearray, respectively. In some embodiments, a pre-limit aperture plate(e.g., primary pre-limit aperture plate 155P_1 (not illustrated)) may beplaced upstream from secondary beam-limit aperture array and anotherpre-limit aperture plate (e.g., pre-limit aperture plate 155P_2 (notillustrated)) may be placed downstream from secondary beam-limitaperture array 155. In such a configuration, one pre-limit apertureplate (e.g., 155P_1) may be configured to block majority of theperipheral secondary electrons from irradiating unintended apertures ofsecondary beam limit aperture array 155, and another pre-limit apertureplate (e.g., 155P_2) may be configured to block any stray peripheralsecondary electrons that may not have been blocked by the firstpre-limit aperture plate, thus mitigating the possibility of occurrenceof crosstalk. It is appreciated that other combinations of the number ofpre-limit aperture plates and their arrangements may be used, asdesired.

In step 940, the trimmed secondary electron beams may be projectedtowards the detection elements (e.g., 140_1, 140_2, and 140_3 of FIG. 3)of the electron detection device to produce images of the probed regionsof the sample.

The embodiments may further be described using the following clauses:

1. An electro-optical system comprising:

-   -   a first pre-limit aperture plate comprising a first aperture        configured to block peripheral charged-particles of a plurality        of secondary charged-particle beams from a sample; and    -   a beam-limit aperture array comprising a second aperture        configured to trim the plurality of secondary charged-particle        beams.

2. The system of clause 1, further comprising a charged-particledetector including a plurality of detection elements, wherein adetection element of the plurality of detection elements is associatedwith a corresponding trimmed beam of the plurality of secondarycharged-particle beams.

3. The system of any one of clauses 1 and 2, wherein a distance betweenthe first pre-limit aperture plate and the beam-limit aperture array is5 mm or less.

4. The system of any one of clauses 1-3, wherein the first pre-limitaperture plate is positioned upstream from the beam-limit aperturearray.

5. The system of any one of clauses 2-4, wherein the first pre-limitaperture plate is positioned downstream from the beam limit aperturearray.

6. The system of any one of clauses 1 and 2, further comprising a secondpre-limit aperture plate.

7. The system of clause 6, wherein the first pre-limit aperture plate ispositioned upstream from the beam-limit aperture array and the secondpre-limit aperture plate is positioned downstream from the beam-limitaperture array.

8. The system of any one of clauses 1-7, wherein the plurality ofsecondary charged-particle beams comprises at least one of secondaryelectrons or back-scattered electrons generated from the sample inresponse to an interaction between a plurality of primarycharged-particle beams and the sample.

9. The system of any one of clauses 1-8, wherein the beam-limit aperturearray comprises a plurality of apertures of different sizes.

10. The system of clause 9, wherein at least two of the plurality ofapertures have similar sizes.

11. The system of any one of clauses 9-10, wherein the plurality ofapertures is arranged in a rectangular, a circular, or a spiral pattern.

12. The system of any one of clauses 1-11, wherein the plurality ofsecondary charged-particle beams overlap to create a crossover area on acrossover plane perpendicular to a secondary optical axis of theelectro-optical system.

13. The system of clause 12, wherein the beam-limit aperture array isplaced on or within a range of positions of the crossover plane andperpendicular to the secondary optical axis.

14. The system of any one of clauses 12 and 13, wherein the range ofpositions of the crossover plane is determined based on a landing energyof the plurality of primary charged-particle beams on the sample.

15. The system of any one of clauses 12-14, wherein the second apertureis centered with the crossover area.

16. The system of any one of clauses 12-15, wherein centers of the firstand the second apertures are aligned with the secondary optical axis.

17. The system of any one of clauses 12-16, wherein the beam-limitaperture array is movable to align an aperture of the plurality ofapertures with the crossover area.

18. The system of any one of clauses 14-17, wherein the beam-limitaperture array is movable along the secondary optical axis based on therange of positions of the crossover plane.

19. The system of any one of clauses 12-18, wherein a plane of the firstpre-limit aperture plate is outside the range of positions of thecrossover plane.

20. The system of clause 19, wherein planes of the beam-limit aperturearray and the first pre-limit aperture plate are within the range ofpositions of the crossover plane.

21. A multi charged-particle beam apparatus comprising:

an electro-optical system for projecting a plurality of secondarycharged-particle beams from a sample onto a charged-particle detector,the electro-optical system comprising:

-   -   a first pre-limit aperture plate comprising a first aperture        configured to block peripheral charged-particles of the        plurality of secondary charged-particle beams; and    -   a beam-limit aperture array comprising a second aperture        configured to trim the plurality of secondary charged-particle        beams,

wherein the charged-particle detector includes a plurality of detectionelements, and

wherein a detection element of the plurality of detection elements isassociated with a corresponding trimmed beam of the plurality ofsecondary charged-particle beams.

22. The apparatus of clause 21, wherein a distance between the firstpre-limit aperture plate and the beam-limit aperture array is 5 mm orless.

23. The apparatus of any one of clauses 21 and 22, wherein the firstpre-limit aperture plate is positioned upstream from the beam-limitaperture array.

24. The apparatus of any one of clauses 21-23, wherein the firstpre-limit aperture plate is positioned downstream from the beam-limitaperture array.

25. The apparatus of clause 21, further comprising a second pre-limitaperture plate.

26. The apparatus of clause 25, wherein the first pre-limit apertureplate is positioned upstream from the beam-limit aperture array and thesecond pre-limit aperture plate is positioned downstream from thebeam-limit aperture array.

27. The apparatus of any one of clauses 21-26, wherein the plurality ofsecondary charged-particle beams comprises at least one of secondaryelectrons or back-scattered electrons generated from the sample inresponse to an interaction between a plurality of primarycharged-particle beams and the sample.

28. The apparatus of any one of clauses 21-27, wherein the beam-limitaperture array comprises a plurality of apertures of different sizes.

29. The apparatus of clause 28, wherein at least two of the plurality ofapertures have similar sizes.

30. The apparatus of any one of clauses 28 and 29, wherein the pluralityof apertures is arranged in a rectangular, a circular, or a spiralpattern.

31. The apparatus of any one of clauses 21-30, wherein the plurality ofsecondary charged-particle beams overlap to create a crossover area on acrossover plane perpendicular to a secondary optical axis of theelectro-optical system.

32. The apparatus of clause 31, wherein the beam-limit aperture array isplaced on or within a range of positions of the crossover plane andperpendicular to the secondary optical axis.

33. The apparatus of any one of clauses 31 and 32, wherein the range ofpositions of the crossover plane is determined based on a landing energyof the plurality of primary charged-particle beams on the sample.

34. The apparatus of any one of clauses 31-33, wherein the secondaperture is centered with the crossover area.

35. The apparatus of any one of clauses 31-34, wherein centers of thefirst and the second apertures are aligned with the secondary opticalaxis.

36. The apparatus of any one of clauses 31-35, wherein the beam-limitaperture array is movable to align an aperture of the plurality ofapertures with the crossover area.

37. The apparatus of any one of clauses 33-36, wherein the beam-limitaperture array is movable along the secondary optical axis based on therange of positions of the crossover plane.

38. The apparatus of any one of clauses 31-37, wherein a plane of thefirst pre-limit aperture plate is outside the range of positions of thecrossover plane.

39. The apparatus of clause 38, wherein planes of the beam-limitaperture array and the first pre-limit aperture plate are within therange of positions of the crossover plane.

40. A method performed by a secondary imaging system to form images of asample, the method comprising:

generating a plurality of secondary charged-particle beams from thesample;

blocking, using a pre-limit aperture plate, peripheral charged-particlesof the plurality of secondary charged-particle beams;

trimming, using an aperture of a beam-limit aperture array, theplurality of secondary charged-particle beams; and

projecting the plurality of trimmed secondary charged-particle beamsonto a corresponding detection element of a charged-particle detector.

A non-transitory computer readable medium may be provided that storesinstructions for a processor of a controller (e.g., controller 50 ofFIG. 1) to carry out image inspection, image acquisition, condenser lensadjusting, activating charged-particle source, beam deflecting,positioning of beam-limit aperture array (e.g., secondary beam-limitaperture array 155), positioning pre-limit aperture plate (e.g.,pre-limit aperture plate 155P), etc. Common forms of non-transitorymedia include, for example, a floppy disk, a flexible disk, hard disk,solid state drive, magnetic tape, or any other magnetic data storagemedium, a Compact Disc Read Only Memory (CD-ROM), any other optical datastorage medium, any physical medium with patterns of holes, a RandomAccess Memory (RAM), a Programmable Read Only Memory (PROM), andErasable Programmable Read Only Memory (EPROM), a FLASH-EPROM or anyother flash memory, Non-Volatile Random Access Memory (NVRAM), a cache,a register, any other memory chip or cartridge, and networked versionsof the same.

It will be appreciated that the embodiments of the present disclosureare not limited to the exact construction that has been described aboveand illustrated in the accompanying drawings, and that variousmodifications and changes may be made without departing from the scopethereof. The present disclosure has been described in connection withvarious embodiments, other embodiments of the invention will be apparentto those skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made as described without departing from the scope of the claimsset out below.

The invention claimed is:
 1. An electro-optical system comprising: afirst pre-limit aperture plate comprising a first aperture configured toblock peripheral charged-particles of a plurality of secondarycharged-particle beams traveling in respective paths associated with asecondary optical axis; and a beam-limit aperture array movable alongthe secondary optical axis and comprising a plurality of apertures,wherein a second aperture of the plurality of apertures is configured tofurther trim the plurality of secondary charged-particle beams.
 2. Thesystem of claim 1, further comprising a charged-particle detectorincluding a plurality of detection elements, wherein a detection elementof the plurality of detection elements is associated with acorresponding trimmed beam of the plurality of secondarycharged-particle beams.
 3. The system of claim 1, wherein a distancebetween the first pre-limit aperture plate and the beam-limit aperturearray is 5 mm or less.
 4. The system of claim 1, wherein the firstpre-limit aperture plate is positioned upstream from the beam-limitaperture array.
 5. The system of claim 2, wherein the first pre-limitaperture plate is positioned downstream from the beam-limit aperturearray.
 6. The system of claim 1, further comprising a second pre-limitaperture plate.
 7. The system of claim 6, wherein the first pre-limitaperture plate is positioned upstream from the beam-limit aperture arrayand the second pre-limit aperture plate is positioned downstream fromthe beam-limit aperture array.
 8. The system of claim 1, wherein theplurality of secondary charged-particle beams comprises at least one ofsecondary electrons or back-scattered electrons generated from a samplein response to an interaction between a plurality of primarycharged-particle beams and the sample.
 9. The system of claim 1, whereinthe plurality of apertures of the beam-limit aperture array comprisesapertures of different sizes.
 10. The system of claim 9, wherein atleast two of the plurality of apertures have similar sizes.
 11. Thesystem of claim 9, wherein the plurality of apertures is arranged in arectangular, a circular, or a spiral pattern.
 12. The system of claim 1,wherein the plurality of secondary charged-particle beams overlap tocreate a crossover area on a crossover plane perpendicular to thesecondary optical axis of the electro-optical system.
 13. The system ofclaim 12, wherein the beam-limit aperture array is placed on or within arange of positions of the crossover plane and perpendicular to thesecondary optical axis.
 14. The system of claim 13, wherein the range ofpositions of the crossover plane is determined based on a landing energyof a plurality of primary charged-particle beams.
 15. A method performedby a secondary imaging system to form images of a sample, the methodcomprising: generating a plurality of secondary charged-particle beamsfrom the sample; blocking, using a pre-limit aperture plate, peripheralcharged-particles of the plurality of secondary charged-particle beams;trimming, using an aperture of a beam-limit aperture array comprising aplurality of apertures and movable along a secondary optical axis of thesecondary imaging system, the plurality of secondary charged-particlebeams; and projecting the plurality of trimmed secondarycharged-particle beams onto a corresponding detection element of acharged-particle detector.
 16. The system of claim 3, wherein thedistance between the first pre-limit aperture plate and the beam-limitaperture array along the secondary optical axis is adjusted such thatthe first pre-limit aperture plate blocks the peripheralcharged-particles from passing through apertures adjacent to the secondaperture.
 17. The system of claim 1, wherein a size of the secondaperture is smaller than the first aperture.
 18. The system of claim 1,wherein the first aperture and the second aperture are aligned with thesecondary optical axis.